The present invention relates to extracting contaminants from soil, particularly to extracting contaminants from low-permeability soil layers, and more particularly to a hybrid joule heating/electro-osmosis process for extracting water soluble and non-aqueous phase liquid contaminants from saturated, low-permeability soil layers.
Contaminants migrating from various types of facilities, accidental spills, and industrial operations threaten health and ground water supplies. Such contamination often covers large volumes of soil underlying several acres of surface area. In view of the high cost of land, limited resources, and the fact that contamination can occur in densely populated areas, such as from leakage of fuel or gas storage tanks or lines, or industrialized areas adjacent dense populated areas, there exists a need to find economical and efficient technologies of remediation for rapid reclamation and rehabilitation of such areas.
Many facilities have suffered contamination of the vadose and saturated regimes by the spilling, or leakage, for example, of dense non-aqueous phase liquids (DNAPLs), such as trichlorethylene (TCE) and other solvents to produce localized sources of contamination. Pump-and-treat methods applied to the source may only dilute the contamination but not remove or even reduce it. Soil removal may be impractical owing to the large volumes that have become contaminated at some sites. Techniques that remediate by either in situ contaminant mobilization and extraction or by in situ breakdown of the contamination into harmless products avoid some of the disadvantages that characterize the pump-and-treat or soil-removal schemes. Steam injection, extraction and air-sparging remediation processes already exist for in situ treatment of moderate-to-high permeability soils. However, the techniques are generally inappropriate for application to low permeability clay layers that have suffered contamination. Cleaning up the higher permeability layers of the soil while neglecting the contamination in adjacent lower permeability formations may permit federal water quality standards to be achieved for several years following cleanup. However, eventually the low permeability contaminated soils will provide a contamination source for the surrounding soil layers, as leaching occurs which results in a decrease of water quality with time.
A process involving ohmic or joule heating of low permeability soil by passing alternating electrical currents through the soil, as a means of in situ contamination mobilization has been studied in some detail (see R. Newmark, Dynamic Underground Stripping Project LLNL Gasoline Spill Demonstration Report 6, UCRL-ID-113521, 1994; and C. R. Carrigan et al., A Fully Coupled Model for 3-D, Partially Saturated Flow and Transport in Soil Ohmically Heated by Application of Multiphase A. C. Electrical Potentials, UCRL-JC-120954, 1996). In addition to mobilizing contamination, ohmic dissipation also provides a potential source of heat for destroying contaminants in situ by hydrous pyrolysis (see K. G. Knauss et al., TCE: Thermodynamic measurements and destruction via hydrous pyrolysis/oxidation, Geol. Soc. Am. Abstr., Vol. 27, No. 6, p. 249, 1995). Electrical heating as a means of either contaminant mobilization or destruction appears to offer significant advantages over steam heating when low permeability clay layers are present. The heating of such soil layers is accomplished by implanting two or more alternating current (AC) electrodes on the edge of the targeted zone of contamination. Two electrodes are the minimum number, but the heating distribution will have little uniformity between the two electrodes. Thus, heating arrangements have used six or more electrodes in a circle to produce more uniform heating of the targeted area. In addition phase-shifting the alternating current applied to each electrode (e.g., the current applied to each electrode of a six-electrode array would be electrically phase shifted by 60 degrees), enhances the heating uniformity of the target zone at the center of the circle (see U.S. Pat. No. 5,330,291 issued Jul. 19, 1994 to W. O. Heath et al., for example). Also, using a six-electrode array, the electrical connections can also involve a three-phase heating arrangement with the six electrodes grouped in three pairs. While the six phase arrangement produces the greatest initial uniform heating, the most serious heating uniformity issues arise when the heating electrodes have been in operation long enough (typically an hour to a day depending on the current applied) to dry out the low permeability soil immediately adjacent to the electrodes. This presents a very serious problem for the ohmic or joule heating technique since groundwater in the soil is a major determining factor of the electrical conductivity of the soil. Thus, drying out of the soil immediately around an electrode is comparable to losing that electrode from the heating circuit, whereby maintaining current carrying capability by resaturation of the soil around the electrodes is necessary.
Another and different electro-remediation phenomenon, known as electro-osmosis, which has been utilized for various applications for about five decades, has been recently considered by researchers as a means of transporting across a porous regime either contamination or solutions intended to mobilize contamination. (See A. P. Shapiro et al., Removal of Contaminant From Saturated Clay by Electro-osmosis, Environ. Sci. Technol., 27, 283-291, 1993; and R. F. Probstein et al., Removal of Contaminants From Soils by Electric Fields, Science, 260, 498-503, 1993.) In addition experiments have demonstrated the ability of electro-osmosis to remove soluble organics from clays. (See Y. B. Acar et al, Phenol Removal From Kaolinite by Electrokinetics, J. Geotech. Eng., 118, 1837-1852, 1992; and U.S. Pat. No. 5,137,608 issued Aug. 11, 1992 to Y. B. Acar et al.), Electro-osmosis is the flow of an ion-containing liquid with respect to a charged surface (i.e., porous medium) in response to an applied electric field across the porous medium. Several models exist that describe the dynamic relationship between the ions in the fluid and the applied field that results in the flow. However, the often-assumed Helmholtz-Smoluchowski Model (See A. T. Young, Electro-kinetic Flow Processes in Porous Media and Their Applications (Chap. 5) in Advances in Porous Media (M. Y. Corapcioglu, ed. 2, Elsevier Amsterdam, pp. 309-395, 1994) provides the simplest and quantitatively adequate understanding of the process. The application of an electric field to a non uniformly distributed charge distribution in a fluid causes the fluid to be more or less dragged through to pore space. Clay pore walls tend to have a negative residual charge which produces the required non uniform charge distribution in the adjacent ion filled fluid. As a remedial technique, the phenomenon has significant potential for restoring low permeability, small-pore, contaminated soils (e.g., clays) since the induced flow does not depend strongly on pore size. On the other hand, for flow in a porous medium that is induced by a simple hydraulic head, there is a strong dependence on pore size with the flux being proportional to the cube of the effective pore diameter. Estimates (see A. P. Shapiro, et al., supra) indicate that electric field strengths of 100 V/m can give rise to electro-osmotic velocities (pore velocities) of about 10 cm/day on a saturated clay. Application of the electro-osmotic process is not new, as pointed out above, and has been applied successfully in civil engineering, separation science and physiological contexts. It has also been used on a large scale for the dewatering of saturated clays to provide a stable foundation for overlying structures. If transport in a low permeability contaminated layer is a necessary component of a remediation scheme, an electro-kinetic mechanism appears to be the only viable possibility.
The present invention involves hybrid processes or techniques for remediating contamination in tight (low permeability) soil layers. These techniques utilize a combination of ohmic or joule heating and electro-osmosis. One technique or process involves in situ destruction of the contamination, while the other involves driving the contamination from the soil layer. In the first hybrid process, electro-osmosis provides the means of transporting oxygen as part of an in situ contaminant destruction technique while electrical dissipation in the soils produces the heating to achieve favorable hydrous pyrolysis reaction rates. In this first hybrid process, a contaminant, such as TCE, is pyrolyzed or xe2x80x9cburnedxe2x80x9d in a hydrous state by providing heat and dissolved oxygen at the same point. In the second hybrid process, described in detail hereinafter, which involves mobilization of the contaminant in the tight or low permeability layer, successful partial desaturation by electro-osmosis will permit the highly effective partitioning of volatile contaminants into the vapor phase which is far more mobile than the liquid phase of a free product or a dissolved contaminant. Heating the resulting partially saturated regime with the same electrodes can drive off the vapor to regions of higher permeability where air sparging or vacuum extraction can be applied. By combining electro-osmosis with ohmic heating, the problem associated with the inherent tendency of ohmic heating to dry the ground out around the electrode well is eliminated. Applying a direct current (dc) voltage difference between an alternating current (ac) ohmic heating electrode and a nearby xe2x80x9csatellitexe2x80x9d dc electrode will induce a groundwater flow toward the ac electrode that can offset the tendency for the ac electrode to dry out the soil thereabouts. The ohmic or joule heating and the electro-osmosis processes can be carried out simultaneously or can be carried out sequentially if the desaturation by electro-osmosis occurs initially.
It is an object of the present invention to provide a process for removing contaminants from saturated, low-permeability soil.
A further object of the invention is to provide a hybrid joule heating/electro-osmosis process for removing contaminants from the soil.
A further object of the invention is to provide for the destruction of contaminants in situ by providing oxygen and heat for hydrous pyrolysis to occur.
A further object of the invention is to provide a joule heating/electro-osmosis process for extracting water soluble and non aqueous phase liquid contaminants from saturated, low-permeability soil layers.
Another object of the invention is to combine joule (ohmic) heating and electro-osmosis processes, wherein partial desaturation of a soil formation or layer is carried out using electro-osmosis to remove a portion of the pore fluid by induction of a ground water flow to extraction wells, whereafter joule heating is performed on the partially desaturated formation.
Another object of the invention is to provide a soil contamination removal process which utilizes joule heating and electro-osmosis simultaneously or sequentially provided desaturation by electro-osmosis occurs initially.
Another object of the invention is to provide a combined joule (ohmic) heating/electro-osmosis process for removing contaminants from a saturated low-permeability soil, wherein transfer of liquid state contaminants to the vapor phase takes place and substantially increases the vapor phase pressure such that the contaminant laden vapor phase is forced out of the low-permeability soil layers into higher permeability soil where such can be removed by conventional processes.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. The invention broadly involves decontamination of soil by the use of a hybrid joule (ohmic) heating/electro-osmosis process. More specifically the invention involves a hybrid joule heating/electro-osmosis process for extracting both water-soluble contaminants and non-aqueous phase liquids (NAPLs) from contaminated, low-permeability soil formations that are saturated. Partial desaturation of the formation is carried out using electro-osmosis, followed by joule (ohmic) heating of the partially desaturated formation. Joule heating of the desaturated formation results in transfer or partitioning of liquid state contaminants to the vapor phase. The heating also substantially increases the vapor phase pressure, and thus the contaminant laden vapor phase is forced out into soil layers of higher permeability where it can be removed by stream stripping or ground water extraction, for example, to capture the contaminants. This hybrid process is more energy efficient than joule heating or steam stripping for cleaning low-permeability formations and the electro-osmosis process can share electrodes with the joule heating process to minimize facility costs. The invention can be effectively utilized, for example, to extract gasoline products or other volatile contaminants from clay layers.